Optical recording media typified by CD (compact disc) and DVD (digital versatile disc) have been widely used for recording various types of information such as video and sound. Recently, further increase of capacity of recording media has been desired in accordance with: enhancement of functions of information processing equipment; and increase of resolution of video equipment.
Reading of information from an optical recording medium and recording of information on the optical recording medium are performed by an optical pickup device irradiating a recording layer of the optical recording medium with a laser beam. Thus, increase of the capacity of the optical recording medium can be achieved by increasing the recording density by decreasing the spot diameter of the beam that is emitted from the optical pickup device. The spot diameter changes depending on the wavelength of the used beam and the NA (numerical aperture) of an objective lens. Specifically, by decreasing the wavelength of the used beam, or by increasing the NA of the objective lens, it is possible to decrease the spot diameter.
In recent years, as an optical recording/reproducing method in which the NA of an objective lens is increased, a near-field optical recording/reproducing method using a SIL (solid immersion lens) is known. In this method, use of an objective lens with an NA exceeding 1 is achieved due to use of the SIL. It is noted that, when the NA of the objective lens exceeds 1, a converging light beam is fully reflected by the exit surface of the objective lens, and hence the light beam is emitted as an evanescent wave, not as a normal light beam. Thus, in the near-field optical recording/reproducing method, the air gap between the exit surface of the objective lens and a surface of an optical recording medium needs to be maintained so as to be shorter than the decay distance of the evanescent wave, in order to propagate the evanescent wave.
Conventionally, in the near-field optical recording/reproducing method using the SIL, information is generally recorded only on one recording layer provided near the surface of the optical recording medium, but methods for recording information on multiple layers have been reported in order to achieve further increase of recording capacity. However, in order to achieve multilayer recording in the near-field optical recording/reproducing method, it is important to focus on each of recording layers at different depths and to compensate an aberration generated at each focal point, in addition to maintaining the above air gap so as to be constant (e.g., see Patent Document 1).
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2003-263770